a) Field of the Invention
The present invention relates to an antifriction bushing for the bearing of a telescopable steering spindle with an inner spindle and an outer spindle which are disposed coaxially and displaceably with respect to one another and, for torque transmission, have a cross section differing from circular. Between the outer spindle and the inner spindle an interspace is provided for receiving the antifriction bushing consisting of a thermoplastic synthetic material or comprising such, and the bushing has an inner surface and an outer surface. The invention furthermore relates to a telescopable steering spindle with an inner spindle and an outer spindle, which are coaxial with respect to one another and (for the torque transmission) have a cross section differing from circular. Between the outer spindle and the inner spindle an interspace is provided in which such an antifriction bushing is disposed.
b) Description of Related Prior Art
Antifriction bushings are employed for friction reduction and/or the clearance or play compensation for example between jacket tubes rotatable with respect to one another and telescopable or between an outer and an inner shaft of a telescopable steering spindle.
Telescopable steering spindles have two coaxial shaft parts having a nonround cross section and which are axially displaceable, and the spindles can transmit a torque. Essential in steering spindles is the freedom from play in the torque transmission, while also having as low a friction level as possible. Telescopable steering spindles are employed in motor vehicles between the steering gear and the generally adjustable steering column. They are intended to compensate for minor changes in the distance between the steering gear and the steering column, such as develop for example due to dynamic loading in a driving operation through distortion of the vehicle body or through movements of the steering gear in a rubber bearing and/or are made possible through a length adjustment of the steering spindle.
The generic prior art is demonstrated in EP 916564 B1. Here, a telescopable steering spindle with nonround profile of the cooperating shaft parts is described. Between the two shaft parts is provided a synthetic bushing with the task of improving the service life of the antifriction properties. In particular, the tendency toward a so-called stick-slip effect is to be decreased. During small movements, this effect causes a force peak in the axial direction due to the difference between static friction and kinetic friction, which peak leads to unacceptable noise development in motor vehicle steering mechanisms.
In telescopable steering spindles according to the prior art, this is not achieved to lasting satisfaction.
As an improvement, DE 102008049825 A1 proposes a selective roughening of the antifriction bushing.
EP 1840193 A2 discloses a sliding connection in which a lubricant with a kinematic viscosity in the range of 1500 to 13,000 mm2/s at 40° C. is employed. Further disclosed is that the contact faces can be coated with a synthetic resin layer.
EP 1840399 A1 also discloses a sliding connection in which a lubricant with a dynamic viscosity in the range of 10 to 210 mm2/s at 25° C. is employed, and the lubricant contains a pressure additive. Onto the contact faces is applied a surface layer, for example of a polytetrafluoroethylene (PTFE).
The improvements proposed in these publications, however, are cost-intensive and complex and to some extent require additional work steps and do not always contribute to a long service life.